Thanks for the advice. I was there this evening at Disney on Ice (a good show if you're into that sort of thing).
The best advice seems to be get their early. We managed to do this, and got parking on the quay just a bit down, in front of Tile Style. They were still letting cars into the (much reduced) on-site car park at that stage (about 45 mins before the start).
I can't imagine what it would be like closer to the start: a bit of a nightmare I suspect, especially if the IFSC car parks fill up.
The new multi-storey card park should be a big improvement.